1. Field of Invention
This invention relates generally to a method and apparatus for detecting the presence of a transmitter, particularly a covert eavesdropping transmitter by detecting the radio frequency (r-f) signal it broadcasts.
2. Summary of the Prior Art
Several methods and devices for detecting the r-f signal emitted by the transmitter of a concealed eavesdropping device are known in the art. Field strength meters measure the intensity of radio transmissions at the frequency to which they are tuned. By scanning a range of frequencies, either manually or with the aid of a computer or the like, an operator can detect signals as peaks in the r-f spectrum.
Signal detection is only half of the task, however. An operator must also assess signal content in order to distinguish between signals from legitimate transmitters (safe signals) and signals from eavesdropping devices. For this reason, field strength measuring devices are often used in conjunction with countermeasures receivers. A countermeasures receiver is simply a radio receiver adapted to meet the special requirements of bug detection. Thus, such a device would typically have high sensitivity and selectivity over the useable r-f spectrum, and multiple-mode demodulation capability. See Van Dewerker, State of the Art of Electronic Surveillance, NWC Commission Studies p. 190 (1976).
The operation of these prior art devices gives rise to several disadvantages. First, the equipment must generally be operated by a highly-trained individual. Second, the entire useable r-f spectrum must be scanned. Insofar as the range of the spectrum which should be searched is considerably broader than the commercial radio band of 88 to 108 MHz, it is easy to imagine that such a task can be extremely time-consuming. Even when a computer performs the signal detection, the human operator must monitor signal content continually over the time period one desires to keep a given area free of eavesdropping devices.
To avoid the need of constant human monitoring, some devices provide for demodulating the intercepted signal and channelling it into the area being checked by means of a loudspeaker. If the signal in question emanates from an eavesdropping device, the device will pick up its own signal, ultimately causing a feedback squeal. One such device is disclosed in U.S. Pat. No. 3,473,127 granted to Williams on Oct. 14, 1969. This remedy has its own disadvantage, however, in that this feedback squeal unavoidably alerts the eavesdropper that detection efforts are being undertaken. Alerting the eavesdropper is undesirable because the eavesdropper may then turn off the eavesdropping device, making the exact location of the bug and the listening post impossible to discern electronically.
Another method directed towards eliminating the need of constant human monitoring involves using a search signal of well defined frequency produced by a loudspeaker in the area to be checked. If a covert eavesdropping device is present, it will pick up and transmit this tone; a countermeasures receiver attuned to the tone frequency can then detect this transmitted signal. Such a method and a device for implementing it are disclosed in U.S. Pat. No. 3,939,420 granted to Risberg et al on Feb. 17, 1976. This method also suffers from the disadvantage that the search signal may alert the eavesdropper. Also, as in the feedback method, the generation of an audible signal in the area to be secured makes the continued operation of these devices during the course of confidential communications impractical.